Medical implants and instruments that are to be used in the human body and be in direct contact with the human tissues need to fulfill several requirements. This is particularly important for implants that are intended for long term or even permanent use. An obvious requirement is that the medical device should be biocompatible, i.e. not release toxic, allergy causing or otherwise noxious substances and should not induce inflammation or rejection from the organism.
An implant also has to fulfill a number of mechanical and chemical requirements. For example, an artificial joint has articulating parts that are exposed to surface-to-surface movement with a large number of load variation cycles. This could lead to abrasion wear, where the joint is slowly worn down and where particles of the implant material is released to the surrounding tissue or even the blood stream, where it may be toxic or may cause inflammation or other complications. The implant therefore has to have great fatigue resistance against that type of abrasion wear. It may also be subjected to large mechanical forces and therefore must be strong enough to tolerate heavy and changing work loads. Since the implant will be placed in an environment of liquids and electrolytes it also needs to be resistant to corrosion.
Additionally, due to an increasing number of young and active implant recipients, and to the longer life expectancy in the population in general, there is an increasing need for high performance medical implants that are longer-lasting. Such implants should be able to withstand many years of wear and other stresses related to being positioned within the body.
Similar requirements also apply to medical instruments. These also have to be biocompatible and are often exposed to large mechanical forces as well as wear processes e.g. during surgical procedures, sterilization etc.
Metals and metal alloys are often used as the structural material for medical implants and devices. For example, titanium, zirconium and cobalt and their alloys, as well as stainless steel, are readily formed and machined into the desired shape. These metals and alloys exhibit a wide range of strengths, hardness and resistances to wear and have been found to comply rather well with many of the described requirements for a body implant. However, when used for longer periods of time or when used in implants with bearing surfaces, e.g. joint implants, where the abrasive wear processes are particularly intense, particles or debris have been found to be shed from these implants. As such debris is disposed to the surrounding tissue it may be encapsulated, induce inflammatory reactions and degradation of the tissue, eventually leading to pain and loosening of the implant. Additionally, the debris may become trapped between the bearing surfaces, thereby further increasing the rate of the wear process.
For example, one material often used in implants is an alloy of cobalt, chrome and molybdenum (CoCrMo). The alloy has been used for implants because of its strength, resistance to wear and corrosion and its biocompatibility. Under conditions of sliding wear or articulation against other bearing surfaces the alloy may, however, start to produce wear debris. It has also appeared that the cobalt content may start to dissolve and diffuse into the bloodstream, which may result in poisoning and injuries of organs. Hypersensitivity to this metal ion debris may also lead to aseptic lymphocytic vasculitis associated lesions (ALVAL), with symptoms of pain and early loosening of the implant. The risk of metal ion shedding is particularly associated with implant devices having metal-to-metal contact surfaces and appears to be lower for implants with metal-to-polymer contact surfaces.
Another material often used for implants and medical instruments is stainless steel, iron (Fe) alloyed with nickel (Ni), chrome (Cr) or Molybdenum (Mo). The material has economical advantages, but it does not have the required corrosion properties, and substances such as nickel may cause allergy.
Harder and stronger materials, such as ceramics, have also been used as the structural material for medical implants. Because of their hardness ceramic materials are more wear resistant and can withstand heavy work loads. Additionally they do not corrode. They are, however, stiffer and more brittle and have therefore, when used as structural bulk material in implants, been more prone to detrimental breaks and fractures.
In the known art a solution to these difficulties has been to use the mentioned metals and metal alloys as a base or substrate material for the main part of the implant, and modify its surface. The metals or metal alloys are ductile and have a high tensile strength, with certain tolerance to plastic deformation, and are therefore suitable as substrate materials. The surface on the other hand is designed to have better tribological properties, such as a higher hardness and strength, to better comply with the requirements on fatigue resistance and wear resistance.
One approach to design a more wear resistant surface has been to modify the surface through processes such as ion implantation, gas nitriding and high temperature oxidation. These processes mainly results in a surface modification of the substrate itself. These approaches have, however, been associated with some limitations, such as creating surfaces that are not hard enough, being too expensive or not possible to use with all kinds of desirable substrates.
Another approach, when designing a material for medical implants, has been to apply ceramic coatings to the surface of a metal or metal alloy surface, either to the whole implant or restricted to the surface areas that are most exposed to abrasive wear. Being hard and strong ceramic coatings, such as aluminum oxide (Al2O3) or zirconium oxide (ZrO2), can provide a fatigue resistant surface to a metal which is resilient but sensitive to abrasion. However, due to the differences in hardness between the substrate and the surface coating and the brittleness of the ceramic, the ceramic has been found to easily crack in this conformation. As a result the coating on these types of materials are inclined to peel off, leaving shed debris and exposing the tissues to the substances of the substrate, and also leading to a dramatic increase the rate of the wear process. The tendency to peel depends both on the mechanical properties of the substrate and the coating, and on how the surface coating is adhered to the underlying substrate.
In order to avoid the tendency of the ceramic coating to crack and peel a solution has been to provide an intermediate layer between the substrate and the ceramic surface coating. The intermediate layer is designed to give a functional, mechanical and/or structural gradient throughout the layer structure, thereby providing good adhesion between the respective layers as well as a more rigid structure. For example, a layer structure having an outer layer comprising aluminum oxide (Al2O3) and an intermediate layer comprising titanium nitride (TiN), titanium carbonitride (TiCN) or titanium oxycarbonitride (TiCNO) has been provided on the surface of substrates comprising, e.g. cobalt chromium (CoCr) based alloys. It has, however, appeared that even this type of layer structure may start to wear down and peel after prolonged use in, e.g. artificial joints or other types of medical implants. There is therefore still a need for medical implants having improved durability relating to, e.g. resistance to abrasive wear.